Photoplethysmography meters and pulse oximeters are known. Using characteristics where hemoglobin in the blood absorbs light in a range from visible light to infrared light, such a meter obtains a change in the intensity of light that is emitted from a light-emitting diode (light-emitting element), passes through or is reflected by a living body such as a finger, and enters a photodiode (light-receiving element) as a photoplethysmographic signal (see, for example, Patent Documents 1 and 2). Extraneous light (for example, sunlight or fluorescent light) other than light emitted from the light-emitting diode sometimes enters the photodiode. Light emitted from the light-emitting diode sometimes enters the photodiode without passing through the living body or being reflected by the living body (stray light). The extraneous light and the stray light are superimposed on, light required to be detected, that is, light that has passed through the living body or been reflected by the living body, and the signal-to-noise (S/N) ratio of a detection signal may be reduced.
Patent Document 1 discloses a pulse oximeter that improves the S/N ratio of a detection signal by performing the subtraction of a reference voltage whose level corresponds to a non-fluctuating (DC) component excluding a pulsating component (pulse wave component). This pulse oximeter includes a first light source (light-emitting element), a second light source (light-emitting element), and a photodiode (light-receiving element).
The light-receiving output (current signal) of the photodiode is converted into a voltage signal on the basis of a reference voltage (Vref 1) by a current/voltage conversion circuit and is amplified by an amplification circuit (amplifier). The amplified voltage signal is converted into a digital signal by an analog-to-digital (A/D) conversion circuit and is input into a computation device. The computation device calculates a reference voltage whose level corresponds to a non-fluctuating (DC) component excluding a pulsating component (pulse wave component) on the basis of the above-described digital signal. The reference voltage is supplied to the current/voltage conversion circuit and is subtracted from a current signal that is a light-receiving output. As a result, a voltage signal corresponding to a pulsating component is output as a detection signal.
Patent Document 2 discloses a pulse wave amplification apparatus that changes a reference voltage for an amplification circuit in consideration of the change in a noise component. This pulse wave amplification apparatus includes a pulse wave detection sensor having a light-emitting element and a light-receiving element, a pulsation pulse generation circuit for generating a pulsation pulse, a conversion circuit for converting the output of the pulse wave detection sensor into an analog signal, an amplification circuit for sampling and holding the analog signal in synchronization with the pulsation pulse, setting a value obtained by the sampling and the holding as a reference voltage, and amplifying a pulse wave signal included in the analog signal using the reference voltage, and a DC store circuit for restoring the DC level of the pulse wave signal output from the amplification circuit in synchronization with the pulsation pulse.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-66119.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2010-166963.
The pulse oximeter disclosed in Patent Document 1 can improve the S/N ratio of a detection signal by performing the subtraction of the reference voltage under conditions where a noise component does not vary even in a case where light on which extraneous light, stray light, or the like is superimposed enters the light-receiving element. However, in a case where a noise component varies in accordance with the change in extraneous light or the change in stray light caused by the movement of a human body, it is difficult for the pulse oximeter disclosed in Patent Document 1 to remove the varying noise component. Accordingly, in a case where a noise component such as extraneous light varies, the S/N ratio of a photoplethysmographic signal may be reduced.
The pulse wave amplification apparatus disclosed in Patent Document 2 can set a reference voltage for each pulsation pulse in accordance with the change in noise component and amplify a pulse wave signal. However, in a case where a noise component varies in the middle of one pulsation pulse cycle, it is impossible to respond to the change in a noise component. Accordingly, in a case where a noise component such as extraneous light varies, the S/N ratio of a photoplethysmographic signal may be reduced.